Oil and petrochemical companies have discovered vast quantities of natural gas in remote locations such as in polar regions and under seas. Transport of natural gas, which consists mostly of methane, is difficult and methane cannot be presently economically converted into more valuable products, such as hydrogen, or into products that are more economically contained or transported, such as liquid fuels, including methanol, formaldehyde and olefins. Typically, the methane is converted to synthesis gas (syngas), which is an intermediate in the conversion of methane to liquid fuels. Syngas is a mixture of hydrogen and carbon monoxide with a H.sub.2 /CO molar ratio of from about 0.6 to about 6. There are many different methods known to convert methane to syngas. These methods include a methane steam reforming process, a carbon dioxide reforming process and the direct partial oxidation of methane.
The direct partial oxidation of methane in accordance with the equation: EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CO+2H.sub.2 (1)
can utilize an ionic conducting membrane reactor or a mixed conducting membrane reactor. In an ionic or mixed conducting membrane reactor, a solid electrolyte membrane that can conduct oxygen ions with infinite selectivity is disposed between an oxygen-containing feed stream and an oxygen-consuming, typically methane-containing, product or purge stream. "Oxygen selectivity" means that the oxygen ions are preferentially transported across the membrane over other elements, and ions thereof. The solid electrolyte membrane is made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconium or analogous oxides having a fluorite or perovskite structure. Such membranes may also be used in gas purification applications as described in European Patent Application Publication Number 778,069 entitled "Reactive Purge for Solid Electrolyte Membrane Gas Separation" by Prasad et al.
At elevated temperatures, generally in excess of 400.degree. C., the solid electrolyte membranes contain mobile oxygen-ion vacancies that provide conduction sites for the selective transport of oxygen ions through the material. The transport through the membrane is driven by the ratio of partial pressure of oxygen (P.sub.O2) across the membrane: O.sup.-- ions flow from the side with high P.sub.O2 to that with low P.sub.O2.
Ionization of O.sub.2 to O.sup.-- takes place on the cathode side of the membrane and the ions are then transported across the solid electrolyte membrane. The O.sup.-- ions are deionized on the anode side to form oxygen atoms which then combine to form O.sub.2 molecules. For materials that exhibit only ionic conductivity, external electrodes are placed on the surfaces of the electrolyte and the electronic current is carried in an external circuit (electrically-driven mode). In mixed conducting materials, electrons are transported to the cathode side internally, thus completing a circuit and obviating the need for external electrodes (pressure-driven mode). Mixed conductors can also be used in the electrically driven mode.
U.S. Pat. No. 4,793,904 to Mazanec et al., that is incorporated by reference in its entirety herein, discloses an ion transport membrane coated on both sides with an electrically conductive layer. An oxygen containing gas contacts one side of the membrane. Oxygen ions are transported through the membrane to the other side, where oxygen reacts with methane or similar hydrocarbons to form syngas. The electrons released by the oxygen ions flow from the conductive layer to external wires and may be utilized to generate electricity.
A mixed conductor-type ceramic membrane has the ability to selectivity transport both oxygen ions and electrons. With this type membrane, it is not necessary to provide an external electric field for removal of the electrons released by the oxygen ions. U.S. Pat. No. 5,306,411 by Mazanec et al., that is incorporated by reference in its entirety herein, discloses applications of a mixed conductor membrane. The membrane has two solid phases in a perovskite crystalline structure: a phase for oxygen ion transport and a second phase for electron conduction. The oxygen ion transport is disclosed as being useful to form syngas and to remediate flue gases such as NO.sub.X and SO.sub.X.
When the ion transport reactor is operated in a pressure driven mode, an anode side reactive purge, in the form of an oxygen scavenging gas, may be introduced to enhance operation of the reactor. The oxygen scavenging gas, such as natural gas, CH.sub.4, methanol, ethanol, hydrogen, or carbon monoxide, reduces the P.sub.O2 on the anode side and enhances the pressure driven O.sub.2 transport through the electrolyte.
A major advantage of using a reactive purge is that it causes a dramatic reduction in the membrane area requirement for a given oxygen removal process because the partial oxygen pressure on the anode is decreased and the transport driving force is increased. However, the energy released by the anode side oxidation reactions can lead to significant exotherms, that is, high temperature rises, in the reactor. If the exotherms are not properly controlled, then the temperature increase that results could damage the ion transport membrane and other reactor components.
If the exotherm is not controlled, the ceramic ion transport membrane may overheat and degrade chemically and mechanically. Further, if portions of the ceramic ion transport membrane heat more than other portions of the membrane, mechanical stresses caused by thermal expansion may also damage the membrane. Still further, the elevated temperatures may damage other reactor components, such as metal to ceramic seals and joints.
Temperature control of a gas separation system is disclosed in U.S. Pat. No. 4,787,919 to Campbell et al., that is incorporated by reference in its entirety herein. Campbell et al. disclose a gas separation system utilizing a polymeric membrane and recite the use of maintaining superheat conditions for the feed gas to avoid condensation of feed gas constituents on the membrane surface. The heat of compression of the feed gas is beneficially used to maintain superheat conditions in a thermally insulated chamber that encompasses the entire reactor. However, in this gas separation system, there are no exothermic chemical reactions analogous to those taking place in a ceramic membrane reactor.
Therefore, there remains a need for a process to maintain the temperature of a ceramic ion transport membrane within a desired temperature range by control of temperature at the membrane level, as opposed to the reactor level, to eliminate the problems inherent in the prior art.